Thermochromic inks change color in response to changes in temperature. Known thermochromic inks often include leuco dyes as the color changing component. Leuco dyes typically exist in two different forms, a colored form and a substantially colorless form. Leuco dyes which change forms in response to local changes in pH are typically used in known thermochromic inks. The change in pH can be brought about in such systems by contacting the leuco dye with a color activator/developer which can induce proton transfer to the leuco dye and thereby cause the leuco dye to adopt its colored form, but similar color changes from substantially colorless to colored can also result from electron transfer and/or proton transfer reactions. From a structural standpoint, the change from substantially colorless to colored is often induced by cleaving a lactone ring to form a more highly conjugated species that absorbs in the visible range. The reverse change from colored to substantially colorless can then be brought about by proton abstraction and reformation of the lactone ring.
Known thermochromic inks often exhibit ‘hysteresis’ associated with the color change, i.e., the color change of written marks made with such inks is reversible. One representative example, U.S. Pat. No. 5,558,699, discloses a thermochromic color-memory composition comprising a homogeneous solubilized mixture of three essential components including (a) an electron-donating color-developing organic compound, (b) an electron-accepting compound, and (c) a reaction medium for controlling the reaction of (a) with (b). The thermochromic compositions of the '699 patent change color with a large hysteresis width (ΔH) of from 8° C. to 80° C. As a result, written marks made using the thermochromic inks typically exhibit a colored state at room temperature, change from the colored state to substantially colorless upon application of heat (i.e., heat can be applied to a substrate to erase written marks previously made thereon), and change back to the initial colored state when cooled below a certain temperature (i.e., the substrate is cooled, thereby “re-forming” the color of the written marks). Thus, known thermochromic inks typically include a reaction medium which can promote or impede the reaction between the leuco dye and the activator at certain temperatures, such that the leuco dye is present in its colorless form at certain temperatures which are typically substantially above room temperature.
The facile reversibility of the color change can be particularly undesirable in certain situations, for example, if the consumer does not wish for the original written marks to be “re-revealed” upon cooling.